1. Field of the Invention
The present invention relates to a semiconductor wafer having a back surface and a front surface which has been coated by chemical vapor deposition (CVD) and a polished or etched back surface, and to a process for producing the semiconductor wafer. The invention also relates to a susceptor for a semiconductor wafer to be placed on during deposition of a layer on a front surface of the semiconductor wafer by chemical vapor deposition (CVD).
2. The Prior Art
Chemical vapor deposition (CVD), in particular deposition of an epitaxial layer on a silicon substrate wafer which has been polished on both sides, involves, inter alia, confronting two phenomena known as “autodoping” and “halo”. In the case of “autodoping”, dopants pass from the back surface of the semiconductor wafer via the gas phase into the deposition gas which is passed over the front surface of the semiconductor wafer. They are then incorporated in the epitaxial layer predominantly in the edge region of the front surface of the semiconductor wafer. This will thereby cause a more or less pronounced, undesired radial fluctuation in the conductivity of the epitaxial layer.
The term “halo” is to be understood as meaning a scattered light effect which is caused by light-scattering structures on the back surface of the semiconductor wafer and manifests itself when the back surface of the semiconductor wafer is irradiated with a focused light beam. The structures mark transitions on the back surface of the semiconductor wafer at which regions with a native oxide layer adjoin regions where an oxide layer of this type is not present. The likewise undesired transitions occur if the removal of the native oxide layer during a preheating phase known as the “prebake” prior to the actual deposition phase was incomplete.
To avoid problems with autodoping, it is proposed in U.S. Pat. No. 6,129,047 to provide slots in the base of the pocket of the susceptor which receives the semiconductor wafer. These slots are arranged at the outer edge of the base. Dopants which diffuse out from the back surface of the semiconductor wafer can be removed from the reactor through the slots without first passing to the front surface of the semiconductor wafer. According to US 2001/0037761 A1, there are small bores in the entire base of the susceptor for the same purpose. Moreover, the removal of the dopant which diffuses out from the back surface of the semiconductor wafer is assisted by sweeping with a purge gas. Both measures also act against halo formation, since they facilitate the removal of the native oxide layer on account of the fact that gaseous reaction products which are formed during dissolution of the native oxide are likewise conveyed away through the holes in the base and with the assistance of the stream of purge gas.
However, the use of the susceptor described is not altogether without problems, since the holes have an effect on the temperature field on the back surface and the front surface of the semiconductor wafer. If the diameter of the holes in the base of the susceptor exceeds a certain size, this has an adverse effect on the nanotopography of the front surface of the semiconductor wafer. The term nanotopography is used to describe height fluctuations in the nanometer range, measured over a lateral area of from 0.5 to 10 mm. The temperature fluctuations caused by the holes, during deposition of the epitaxial layer on the front surface of the semiconductor wafer, lead to locally differing deposition rates and ultimately to the abovementioned height fluctuations. To avoid this problem, it is proposed in US 2001/0037761 A1 to limit the diameter of the holes and to make the temperature field more uniform by adapting the power of the lamp heating.
However, these measures are only effective with regard to the front surface of the semiconductor wafer. As the inventors of the present invention have established, the nanotopography values of the back surface of the semiconductor wafer even become worse the smaller the diameter of the holes in the susceptor is selected to be. The presence of the holes means that the temperature field on the back surface of the semiconductor wafer remains so uneven that local etching, caused by purge gas, such as for example hydrogen, and local deposition, caused by deposition gas reaching the back surface of the semiconductor wafer, occur. Both phenomena have an adverse effect on the nanotopography of the back surface, which cannot be tolerated since even unevenness on the back surface of the semiconductor wafer can cause focusing problems for the stepper during fabrication of electronic components on the front surface of the semiconductor wafer.